Researchers from the A1 collaboration group at the Institute of Nuclear Physics of the Johannes Gutenberg University Mainz (JGU), in collaboration with scientists from China and Japan, have successfully prepared hydrogen-6, one of the most neutron-rich isotopes, using electron scattering for the first time. The experiment, carried out at the spectrometer facility at the Mainz MicroAccelerator (MAMI), provides a new method for studying light, neutron-rich nuclei.These findings provide new insights and pose significant challenges to existing models of multinucleon interactions.

"This measurement was made possible thanks to the unique combination of the excellent quality of the MAMI electron beam and the three high-resolution spectrometers of the A1 collaboration," emphasizes Professor Josef Pochodzalla of the Institute of Nuclear Physics at Guggenheim University in Japan. Researchers from Fudan University in Shanghai, China, and Tohoku University in Sendai and the University of Tokyo in Japan participated in the experiment.

The experimental work was led by doctoral student Shao Tianhao and has been published in Physical Review Letters.

Nuclear structural limits of extremely neutron-rich systems

One of the most fundamental questions in nuclear physics is how many neutrons can be combined in an atomic nucleus, and what is the number of protons. For the basic isotope hydrogen, which contains only one proton, in addition to the familiar deuterons and tritium nuclei, a number of neutron-rich isotopes have also been observed, ranging from ⁴H to ⁷H.

The setup of three high-resolution spectrometers in the A1 experimental hall is used to detect ⁶H. Photo credit: Ryoko Kino, Josef Pochodzalla

The extremely heavy hydrogen isotopes ⁶H (made up of one proton and five neutrons) and ⁷H (one more neutron) have the highest neutron-to-proton ratios known to date. They are a unique system that answers this question. However, experimental data on these exotic nuclei are scarce, and the results remain controversial. In particular, there has been controversy over whether the ⁶H ground state energy is high or low.

The A1 collaboration, working with scientists in China and Japan, has developed a new method for producing ⁶H. In this method, an electron beam with an energy of 855 megaelectronvolts (MeV) strikes a ⁷Li target, producing ⁶H through a two-step process: First, protons in the lithium nucleus are resonantly excited due to their interaction with electrons, and rapidly decay into neutrons and positively charged pions.

If the neutron then transfers energy to another proton in the nucleus, it can join the remaining nucleus to form the neutron-rich hydrogen isotope ⁶H. The pions and protons leave the nucleus and can be detected simultaneously with the scattered electrons by three magnetic spectrometers. To achieve sufficient throughput for this rare process, the electron beam passes through a 45-mm-long, 0.75-mm-thick lithium plate along one side. This is very rare because electron scattering experiments typically use targets that are very thin along the beam axis, allowing the electron beam to strike a wide surface perpendicular to its direction of propagation.

This particular setup benefits from MAMI's excellent beam quality, especially its highly focused and stable electron beam. Another challenge is handling the lithium itself, as the material is extremely chemically reactive, mechanically brittle, and temperature-sensitive.

During the four-week measurement campaign, as expected, approximately one event was observed per day. In one of MAMI's rare experiments, three high-resolution spectrometers in the A1 experimental hall operate simultaneously in coincidence mode, allowing three particles to be detected simultaneously. This sophisticated device achieves unprecedented precision while maintaining extremely low background noise.

The new measurements provide a clear signal for ⁶H, which has an extremely low ground state energy, indicating that the interaction between neutrons in 6H is stronger than expected from recent theoretical calculations. This result challenges our understanding of multinucleon interactions in systems with extremely high neutron abundance.

Compiled from /ScitechDaily